OpenACS 4.6.1 is PG 7.3.1 compatible. Upgrade to that version if you haven't already done so. If you are at 4.6.1 and find bugs please report them in the bugtracker.
4.6.1 hasn't really been released. you can check out the
head of the 4.6 branch with -r oacs-4-6 to get the patched
version. Should be released end of next week if you would rather wait.